Our goals are to elucidate the molecular mechanisms that control the assembly and disassembly of actin filaments in cells and to understand how actin assembly dynamics contribute to cell movement. The creation of free barbed ends of actin filaments is a critical determinant of actin assembly, and capping those ends is necessary to produce force and movement at membranes. Here, we investigate the function of the heterodimeric barbed-end capping protein (CP) and a set of membrane- associated proteins that contain a conserved CP-binding motif, called CPI, but are otherwise unrelated. CARMIL, which contains the CPI motif and a second CP-binding motif, called CSI (CARMIL-specific interacting), is a potent inhibitor of CP with the ability to create free barbed ends by uncapping capped filaments. Uncapping is important because the turnover rates of CP and actin filaments in cells are faster by orders of magnitude than those observed with purified proteins in vitro. Other CPI-motif proteins, including CD2AP, Cin85, CKIP-1, WASHCAP(FAM21) and CapZIP bind CP but inhibit less well, suggesting that they may target active CP to membrane compartments. Aim 1: How Do Regulators of Capping Protein Work? To understand how CP regulation works, we defined the actin-binding sites on CP, and we produced crystal structures for CP in complex with CP-binding proteins. CPI motifs bind to a common site on CP at a distance from the actin-binding sites. CARMIL binding causes an allosteric change in the conformation of the actin-binding sites. Now, we ask whether the cellular function of the various CPI-motif proteins is to inhibit CP or to recruit active CP to a location in the cell. In vitro, we will compare the abilities of the proteins to bind and inhibit CP in biochemical assays with purified components. In cells, we will determine how targeting and incorporation of CP into the actin cytoskeleton depends on the CP-interacting proteins and how their interaction with CP affects local actin assembly and movement. Aim 2: How Does CARMIL1 Function in Cells? CARMIL is important for cell migration and other actin-related functions in metazoan cells. Vertebrates express three conserved CARMIL genes. We discovered that CARMIL1 and CARMIL2 are both important for cell migration and that they have distinct non-overlapping functions in a single migrating cell. CARMIL1 interacts with the dual-GEF Trio, activates Rac1, and stimulates lamellipodia formation. We plan to investigate the molecular basis of these phenotypes and interactions by a combination of localization, biochemical, knockdown, and expression approaches in human cultured cells and zebrafish.